1. Field of the Invention
The present application relates to an over-current protection device, and more particularly to a radial-leaded over-current protection device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Because the resistance of a conductive composite material having positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at a normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state, i.e., trip, so as to suppress over-current and protect the cell or the circuit device.
In general, the PTC conductive composite material contains crystalline polymer and conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer. The crystalline polymer is usually a polyolefin polymer or a fluoropolyolefin polymer such as polyethylene. The conductive filler is usually carbon black.
The electrical conductivity of the PTC conductive composite material depends on the content and type of the conductive filler. In general, the resistivity of the PTC conductive composite material containing the carbon black as the conductive filler is not low enough, and therefore the composite material of large resistivity is not suitably applied to miniature devices. Because carbon black is of relatively low electrical conductivity, a large hold current of the device containing carbon black is hard to be attained. The hold current indicates a maximum current that the PTC device can endure before trip at a specific temperature. To develop a device of a large hold current, conductive filler of a lower resistivity than carbon black has to be used. However, even the PTC conductive composite material of a resistivity below 0.2 Ω-cm may be achieved by using metal conductive filler, it often loses voltage endurance.
With the advancement of miniaturization of devices, it is difficult to further decrease the resistance of a miniature device and sustain large hold current simultaneously. In particular, in an attempt to decrease the entire resistance of the radial-leaded over-current protection device, it has to consider not only the resistance of the PTC device but also the composition, shape and size of the external electrode leads associated with the PTC device.